Module 4 - Safety
Stability and Seaworthiness
The yacht's weight acts down through its centre of gravity, G, while buoyancy acts up through the centre of the underwater volume, B. As a stable monohull heels, the underwater shape changes and the horizontal separation of those force lines forms a righting lever, GZ. The resulting righting moment depends on the yacht's displacement and GZ; it changes with heel and loading and cannot be inferred from the upright feel alone.
Weight added high raises G and can reduce the ability to recover from a knockdown. Loose gear can shift to the low side. Liquid with a free surface in a slack tank, bilge or on deck moves as the yacht heels and produces a virtual loss of stability. The effect depends on tank geometry, subdivision, displacement and the stability data—not simply whether a tank is 'half full'. Follow the yacht's loading and tank guidance rather than applying a universal full-or-empty rule.
Water aboard causes several connected problems: added weight, free surface, reduced freeboard and possible progressive downflooding. Keep bilge suctions, cockpit drains and freeing ports clear; investigate unexplained water immediately. Close openings marked to remain shut at sea and secure washboards or hatches before rough conditions, while preserving the ventilation or system openings that must remain functional.
Seaworthiness is a continuing state. Secure heavy items for large heel angles, inspect seacocks, hoses and skin fittings, keep suitable bungs accessible, and know the bilge-pump limitations. A rising bilge level, new list, slower roll, blocked drain or water near an off-centre opening is evidence to reduce load, control ingress, change the plan and seek help before the margin disappears.
- Centre of gravity (G) — point through which the vessel's weight acts downward
- Centre of buoyancy (B) — centre of the underwater volume; its line of action changes as the yacht heels
- Righting moment — displacement multiplied by the righting lever GZ for the loading and heel considered
- Heavy gear aloft raises G, reducing stability
- Free surface — a virtual loss of stability governed by tank or flooded-space geometry and displacement
- Flooding adds weight, free surface and downflooding risk while reducing freeboard
- Reduce and secure topweight within the yacht's loading information
- Keep drains clear and secure required closures before rough conditions
| Evidence | Why it matters | Immediate question |
|---|---|---|
| New water in the bilge | Added weight and free surface | Where is it entering, at what rate, and can pumps keep up? |
| Heavy item moved high | Raised centre of gravity | Can it be removed, lowered or secured within the yacht's loading guidance? |
| Blocked cockpit drain | Trapped water and reduced freeboard | Can it be cleared without exposing crew or opening a new flooding path? |
| Persistent list | Lost margin on one side | Is weight shifting, water accumulating or a compartment flooding? |
| Opening nearing the water | Progressive downflooding risk | Can it be secured now and the heel or route reduced? |
Watch for this: Monitor ingress as a rate, not a snapshot. One litre every ten seconds becomes 360 litres per hour and 1,080 litres in three hours; a seemingly small leak can create more than a tonne of mobile water.
Optional quick check
Section 9 of 10
The bilge pump is cycling more often and a new list is developing. What is the strongest first command response?
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